1. Field of the Invention
The present invention is related to charge pumps, and more particularly, to digitally controlled active feedback charge pumps.
2. Related Art
A charge pump regulator is a DC/DC switching converter that converts a lower input voltage and regulates to a higher output voltage or vice versa. The advantage of a charge pump is that it stores energy in a relatively cheaper capacitor instead of in an inductor. Commonly used topologies in most of the commercial integrated circuits are “skip” mode and “linear” (constant frequency) mode.
The voltage regulation of the charge pump is controlled using both the “skip” and “linear” modes. The “skip” mode operation is depicted in FIGS. 1A-1B.
The circuit includes two capacitors, a pumping capacitor Cpump and a reservoir capacitor Cres. These storage elements provide current to the load at the output. The output voltage Vout is set by the ratio of resistors R1, R2 and is governed by Vout=Vref*(R1+R2)/R2. In FIG. 1B, during φ1, both transistors M1, M3 are turned on, and transistors M2, M4 are turned off. Node “x” is charged to Vin. During φ2, transistors M2, M4 are turned on and transistors M1, M3 are turned off. Cres is being charged to the desired output voltage. The same charging sequence continues until a comparator 101 detects an output higher than the desired voltage, and then disables a clock generator 102. The circuit skips switching (i.e., idles) until the output voltage Vout drops lower. The clock generator 102 is then reactivated. The same principle applies to FIG. 1A except that the transistors M3, M4 are replaced by diodes D1, D2 respectively (this circuit is known as a Dickson charge pump).
In FIG. 2, a Dickson charge pump operating in a constant frequency mode is illustrated. When the Dickson charge pump operates at a constant frequency, there are no skip (idle) cycles. The output of the comparator 101 feeds into a control circuit 202, which generates a DC voltage to control the on-resistance of transistor M2 under different loading conditions. Hence, voltage regulation at Vout can be achieved by adjusting the I*R drop across the transistor M2.
A third regulation scheme is to combine the “skip” and “linear” mode to form an active-control circuit. This is depicted in FIG. 3. In this control scheme, skip cycles are inserted, in addition to the use of resistance control.
FIG. 4 shows an operating principle of the Dickson charge pump with active-cycle control. Let Qc be the charge transferred to the Cpump during a charging cycle and Qp be the charges delivered to the load and to the Cres during a pumping cycle. Mathematically, |Qc|=|Qp|. Hence the average current during the time tp is |Ic|=|IP|, Ip is the current passing through D2, and IL is the load current, where
      I    p    =            I      L        +          I      L        +                            (                                    I              L                        *            tw                    )                tp            .      IL is also the charging current to Cres. (IL*tw) is the reserved charge to supply IL in idle.
Vout can be expressed as follows:Vout=(Vmbat−VF−IcRn)+(Vcc−VF−IpRp)where Rp, Rn are the on resistances of M2, M1 respectively, Vmbat is a battery voltage, VF is the forward junction voltage of the diodes D1, D2.
  Vout  =      Vmbat    +    Vcc    -          2      ⁢      VF        -                            I          L                ⁡                  (                      2            +                          tw              tp                                )                    ⁢              (                  Rp          +          Rn                )            
Here, the
  (      2    +          tw      tp        )term is the skip control, and the (Rp+Rn) term is the resistance control.
The above equation shows how the output voltage can be controlled by adjusting the resistance of M1, M2 and by inserting skip cycles. Note that Vmbat can be the same as Vcc if the anode of the diode D1 is tied to the source of M2.
The problem with the approaches above is as follows: if analog feedback is used, there are often stability issues that need to be addressed fairly carefully. Additionally, pulse width modulation (PWM) schemes can be device-intensive in terms of implementation. Also, feedback control for pulse width modulation circuits frequently has stability problems, and needs to be very carefully designed. For example, such circuits may require lead-lag compensation, or may require transistors that number in the tens or even hundreds to achieve the pulse-width modulation with stable feedback control.